i965/vec4: Improve src_reg/dst_reg conversion constructors.

i965/vec4: Improve src_reg/dst_reg conversion constructors.

This simplifies the src_reg/dst_reg conversion constructors using the
swizzle utils introduced in a previous patch.  It also makes them more
useful by changing their semantics slightly: dst_reg(src_reg) used to
set the writemask to XYZW if the src_reg swizzle was anything other
than XXXX, which was almost certainly not what the caller intended if
the swizzle was non-trivial.  After this patch the same components
that are present in the swizzle will be enabled in the resulting
writemask.

src_reg(dst_reg) used to set the first components of the swizzle to
the enabled components of the writemask and then replicate the last
enabled component to fill the swizzle, which, in cases where the
writemask didn't have exactly the first n components set, would in
general not be compatible with the original dst_reg.  E.g.:

| ADD(tmp, src_reg(tmp), src_reg(1));

would *not* do what one would expect (add one to each of the enabled
components of tmp) if tmp didn't have a writemask of the described
form (e.g. YZ, YW, XZW would all fail).  This pattern actually occurs
in many different places in the VEC4 back-end, it's a wonder that it
hasn't caused piglit failures until now.  After this patch
src_reg(dst_reg) will construct a swizzle with each enabled component
at its natural position (e.g. Y at the second position, Z at the
third, and so on).  The resulting swizzle will behave like the
identity when used in any instruction with the original writemask.

I've manually verified that *none* of the callers of both conversion
constructors were relying on the previous broken semantics.  There are
no piglit regressions on any generation.

Reviewed-by: Matt Turner <mattst88@gmail.com>